Cathode



April 5, 1966 P. w. STUTSMAN CATHODE Filed April 25, 1963 INVENTOR PAUL W. STUTSMAN BY Flt/1] T 9% AGENT United States Patent 3,244,930 CATHODE Paul W. Stutsman, Needham, Mass, assignor to Raytheon Company, Lexington, Mass., a corporation of Delaware Filed Apr. 25, 1963, Ser. No. 275,744 Claims. (Cl. 313-311) This invention relates generally to gaseous electron discharge devices and more particularly to grid-controlled gaseous discharge devices of the cold cathode type.

The device of the present invention is particularly well suited for use in arcdischarge applications requiring high hold-0E voltage, high pulse current handling capability, extremely fast switching response and a long and reliable operating life. An example of such an application is the arc discharge switching requirement for electronic ignition systems in gas turbine engines where the operational efliciency is dependent to a large extent upon the required time for the switching tube to be triggered to full current conduction.

Experience indicates that cold cathode gas filled trigger tubes known in the prior art have generally been criticized for their short operating lives. Early failures are believed to stem from the fact that the heavy arc current is conducted through the active cathode material creating excessive ohmic heating and resulting in rapid deterioration of the cathode. The inventor has observed that known cold cathodes that are formed by compressing powdered mixtures of conductive metal and emitter material into pills have been particularly poor with respect to current handling capacity and operating life.

It is a principal object of the present invention to pro vide a cold cathode gaseous discharge device having an improved sputter-resistant composite cathode which is capable of conducting large current pulses with very little heating of the active cathode material and which has a long operating life.

In accordance with the present invention, an improved cold cathode discharge device is provided having a composite col-d cathode which is formed by fusing an homogeneous mixture of powdered metal and a halogen salt of an alkali metal in an electrically conductive metal cup. In the preferred method of making the cathode, a mixture of sodium chloride and nickel powder are fused in a nickel cup in an atmosphere of air at temperature of 1100 C. to form an amorphous slag. The nickel powder inhibits and effectively precludes the formation of large salt crystals during the cooling of the fused mass and at the same time serves to anchor the resultant cinderlike block of active cathode material into the cup. At no time during the process is the fused mixture com pressed in the nickel cup. In practice the cup is filled several times and the heating process is repeated to assure complete filling of the cup with the hard, durable sputter-resistant cathode material.

In accordance with a further feature of the invention, the nickel cathode cup is formed with internal radial vanes which greatly minimize evaporation of the low work function active cathode material and accordingly extend the cathode life.

In a preferred embodiment of the invention, the anode of the gas-filled discharge tube is provided with an active emitting block and made substantially the same as the above-described composite cathode. In applications where the anode swings negatively with respect to the cathode, the active material on the anode becomes emissive and anode erosion due to sputtering is greatly minimized.

Other structural and operational features of the present invention will be described in further detail in connection with the following descriptions of the drawing in which:

FIG. 1 shows in partial perspective view a completely "ice assembled tube in accordance with the teachings of the present invention;

FIGS. 2 and 2a are plan and sectional views, respectively, showing the improved composite cathode provided in the tube as illustrated in FIG. 1;

FIG. 3 illustrates in graphical form the typical current transient switching response of the trigger tube as shown in FIG. 1; and

FIG. 4 is a schematic diagram of a typical self-firing and self-resetting pulse trigger circuit utilizing the tube in FIG. 1.

Referring now to FIG. 1, there is shown an electron discharge device 10 including cathode 11, grid 12 and an anode 13 each mounted within glass envelope 14. Anode 13 is directly connected to an external anode connector pin 17; cathode 11 is connected to base pins 18, and grid 12 is electrically connected to four individual base pins 20 as shown. Plural leads to the grid and cathode elements, respectively, are utilized to minimize lead inductance and to reduce lead resistance in the cathode circuit. Envelope 14 is filled with a pressurized inert gas and in the preferred embodiment the inventor has obtained excellent results with argon.

As indicated above, one of the featured operational advantages of the present tube resides in its long operating life which is attributable primarily to the improved composite cathode structure illustrated in FIGS. 2 and 2a. The cathode includes a metal cup 30, preferably formed of nickel, with internal radial vanes 31. Active material for the cathode is formed by filling the separate quadrants of the cup with a mixture of sodium chloride and nickel powder and fusing the mixture in an atmosphere of air at a temperature of approximately 1100 C. The inventor has obtained excellent results with a mixture of 87 /2 percent sodium chloride and 12 /2 percent nickel powder 'by weight. After fusion, the amorphous slag and cathode cup are allowed to cool at room temperature. The resultant active cathode material is a hard cinder-like block which is sputter-resistant, which is capable of handling very large pulse currents (e.g., 1000 amperes for a cup having an outside diameter of 0.3l2 inch, a depth of 0.15 inch and a wall thickness of 0.005 inch) and which has an excellent operating life.

In practice, the inventor has found that the ratio of sodium chloride to nickel powder may be varied over a limited range without seriously effecting the operating life of the cathode but that the percentage of nickel powder by weight should preferably be no higher than 20 percent. The inventor has further found that when pure sodium chloride is used for the active cathode material, large salt crystals are formed and the cathode is rapidly deteriorated by removal of these crystals from the cathode surface due to the kinetic forces produced by the anode-cathode arc. The addition of nickel powder serves to anchor the salt to the cathode cup and prevent the formation of large salt crystals. Excessive amounts of nickel powder have been found to greatly shorten the tube life by causing more of the arc current to flow through the active material of the cathode rather than through the peripheral edges and radial vanes of the nickel cup.

Halogen salts of other low work function alkali metals such as potassium or cesium may be used in place of sodium chloride. Sodiumchloride has an important advantage in that it can be fused with the nickel powder in open air without presenting any health hazards, whereas the fumes of cesium and potassium are poisonous.

In accordance with another feature of the present invention, the anode 13 is made similar to the cathode shown in FIGS. 2 and 2a except that the radial vanes are omitted. By providing the anode with an emissive material, corrosion of the anode sputtering is greatly minimized. 'Ihis is due to the fact that in applications where the anode circuit may oscillate to a negative voltage, the anode will emit and greatly minimize erosion due to sputtering.

In a production model of the above described invention, the grid ring 12 is formed 'of 0.02 inch nickel Wire and is spaced approximately 0.015 inch from the cathode. The envelope, having dimensions of approximately 0.7 inch diameter and a height of 1.9 inches over-all, is filled with argon gas to an absolute pressure of approximately 61 centimeters of mercury. The anode 13 is spaced approximately 8 mm. from the cathode 11. For anode to cathode voltages less than the gap breakdown voltage of 3000 volts, the tube firing is controlled by the grid. Application of voltages in excess of 300 volts positive between the grid and cathode initiate a glow discharge between the grid wire and a cathode vane or sidewall of the nickel cup. This glow discharge produces electrons and photons which ionize the argon gas and cause an arc to form between -'the anode and cathode. The high current are causes local dissociation of the sodium chloride at thesurface 32 of the cathode 11. The high voltage at the anode'(e.g., 2500 volts) causes streamer formations as the initial stage in the arc breakdown between anode and cathode. The formation of streamers 'betweenthe anode and cathode produces additional photoionization in the argon and this completes the arc breakdown between anode and cathode. The sodium vapor produced by the local dissociation .of-the sodium chloride "lowers the work function of the cathode resulting in a low arc drop between anode and cathode, while the chlorine gas maintains the electrical properties of the argon gas, keeping the breakdown voltage at its rated value.

During the passage of the arc,-the top peripheral surface of the nickel cup and the radial vanes remove heat from the sodium vapor and serve to greatly minimize evaporation of the sodium. As a consequence the cup vanes and edge surfaces are coated with a thin layer of sodium compound. This thin layer of sodium compound functions as the principal emitting surface of the cathode and hence causes the major portionof the arc current to be shunted through the nickel vanes and the peripheral edge of the cup rather than through the active cathode material 32. Ohmic heating of the cathode active material is thereby greatly reduced and, accordingly, the cathode life is greatly increased.

The above-described trigger tube is capable of delivering to a work load'one to two joules of energy at 2400- p 2800 volts with a switching time of one microsecond at a repetition rate of ten'pulses per second. Such tubes have been operated satisfactorily for more than seven million pulses.

It should be'noted that the pressure of the gas'filling the tube envelope is an important prarameter since'it largely controls or determines the breakdown potential of the tube and hence the working voltages. The breakdown potential of the tube is also effected by thechlorine gas produced by dissociation of the sodium chloride. The chlorine gas tends to increase'the hold-off or stand-ofl? voltagebecause of its high electron capture cross-section. The use of a relatively high gas pressure (e.g. 61 centimeters of argon) assists the glow to are transition period in that less power is required'from the anode-cathode circuit ,to establish the arc. Higher gas pressure minimizes erosion of the electrodes due to'sputtering and evaporation and also improves the stability of the tube characteristics by. providing suflicient gas to minimize the effect of gas clean-up.

Ingeneral it can be said that the breakdown potential or stand-off potential of the tube is increased with in- 4 creased gas pressure and/or with increased spacing between the grid and cathode.

FIG. 3 shows a typical current switching response of the tube in FIG. 1. Typical switching times for an anode to cathode voltage of 2600 volts and a peak current 1; of 520 amperes are T =0.6 microsecond, T =2 microseconds and T =3.3 microseconds. The negative ourrent overhoot following the first pulse and identified in FIG. 3 as I is due'to inductancein the associated circuit. This may be minimized by choice of components and proper design.

FIG. 4 is a schematic diagram of a typical D.C. operated self-firing and resetting pulse switching circuit utilizingt-he tube in FIG. 1. Capacitor C is charged through R and R 'by the 3200 volt 'D.C. supply as shown. Grid voltage is provided by divider network R and R Capacitor C is charged by the potential at the junction of R and R and when the voltage on C reaches the critical grid voltage, the grid-cathode glow discharge is initiated, which in turn initiates the anode-cathode arc discharge as described above. Capacitor C is discharged through R;, and the tube 10. Current conduction through '(the tube is terminated when the current from C is insuflicient to maintain the arc discharge. In like manner, capacitor C discharges through the grid-cathode circuit until the grid voltage drops below that necessary to maintain the glow discharge.

'While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood-by those skilled in the art that various detailed changes may' be made therein without depart- .ing from the spirit and scope of the invention.

What is claimed is: 1. In a gaseous discharge device, a cathode comprising: an electrically conductive nickel cup containing a substantially uncrystallized fused mixture consisting of powdered nickel and sodium chloride. In a gaseous discharge device, a cathode comprising: a nickel cup containing a substantially uncrystallized fused mixture of 12 /2 percent powdered nickel by weight and 87 /2 percent sodium chloride by weight. 3. In a gaseous discharge device, a cathode comprising: a nickel cup havin-ginternal radial partitioning vanes and containing a substantially u-ncrystallized fused mixture consisting of powdered nickel and sodium chloride. 4. 'In a gaseous discharge device, a cathode comprising: a nickel cup [having internal radial partitioning vanes and containing a substantially uncrystallized fused mixture consisting of a powdered nickel and a halogen salt of an alkali metal. 5. In arr-electron discharge device, a cathode comprising:

a nickel cup containing an electron emissive body embodying up to about 20% by weight of nickel with the remainder thereof being sodium chloride.

References Cited by the Examiner UNIT ED STATES PATENTS 2,604,603 7/ 1 952 McLind-en 313193 3,065,371 1 1/1962 Stotsman 313-197 3,076,913 2/1963 Firth 313-- 3,110,081 -1l/l963 Hendriks 2925.17 3,113,370 12/1963 Lehman 2925.17

GEORGE N. WESTBY, Primary Examiner.

D. E. SRAGOW, Assistant Examiner. 

1. IN A GASEOUS DISCHARGE DEVICE, A CATHODE COMPRISING: AN ELECTRICALLY CONDUCTIVE NICKEL CUP CONTAINING A SUBSTANTIALLY UNCRYSTALLIZED FUSED MIXTURE CONSISTING OF POWDERED NICKEL AND SODIUM CHLORIDE. 